The human eye can only see that portion of the electromagnetic spectrum generally referred to as visible light (e.g., radiation having a wavelength of about 400 to about 700 nanometers). All other portions of the electromagnetic spectrum (e.g., near infrared radiation having a wavelength of about 700 nanometers to about 5000 nanometers) typically require some type of optical intervention and assistance in order to provide an image that the human eye can clearly discern.
Of particular interest are modern imaging technologies used for detecting objects in the infrared portion of the electromagnetic spectrum, especially the near infrared portion. These imaging technologies have particular application for uses such as night vision goggles (NVG's) and heads-up displays for the military, as well as civilian police departments. These devices are particularly useful for imaging objects at night when there is little or no visible light present; although there is an abundance of near infrared radiation available. The near infrared radiation emitted by an object is typically due to heat being radiated from the object (e.g., a human, an automobile, a tank, an airplane, and the like). Therefore, by way of a non-limiting example, police department helicopters can track a suspect who is fleeing from ground units at night due to the fact that the suspect, when viewed in the infrared spectrum, will appear to be “illuminated” (because he/she is radiating body heat) whereas the surrounding ground will appear “darkened” (because it is not radiating any significant amounts of heat).
In order for this night vision technology to achieve its intended function, it is generally recognized that some sort of filtering mechanism must be employed to block out those parts of the electromagnetic spectrum that are undesired. Therefore, if the viewing device is intended to allow the operator to view objects in the near infrared portion of the electromagnetic spectrum, it would not make sense to allow visible light, ultraviolet light, or even mid-infrared and far-infrared light, to be transmitted through the filtering mechanism. If this were to happen, the resolution of the resulting image would be compromised. According, many night vision viewing devices employ some sort of infrared filter mechanism to only allow a specific wavelength range (or ranges) of near infrared light to be transmitted through the filter mechanism.
One type of optical filter that has been used for night vision filter applications is generally referred to as a comb filter. A comb filter generally consists of a thin, flat plate of a transparent anisotropic substance (e.g., glass or polymer substrate) coated (e.g., by vacuum deposition, ion sputtering, and so forth) with a thin interference film of dielectric material (e.g., an oxide such as silicon dioxide or titanium dioxide). The dielectric coating permits certain wavelengths bands to pass through the substrate, while other wavelengths bands are reflected. Therefore, a comb filter typically consists of many alternating “pass” bands, where the light wavelength is transmitted and “stop” bands, where the light wavelength is not transmitted (i.e., reflected), thus giving a typical comb filter transmittance plot a “spiky” appearance, not unlike the teeth of a hair comb.
Of particular interest are rugate coatings which are generally defined as optical interference films wherein the refractive index of the film continuously and periodically grades as a function of the film's optical thickness.
A special type of comb filter system consists of a pair of complementary comb filters. By “complementary” it is meant that the pass and stop bands of the first comb filter are reversed on the second comb filter. Thus, a complementary pair of comb filters has the property that one comb filter blocks light wavelengths that are transmitted by the other comb filter, and vice versa. In this manner, a greater number of highly discrete and desirable light wavelengths can be transmitted with a complementary comb filter pair than with only a single comb filter.
Typically, each comb filter of the complementary pair is fabricated in its' own separate deposition run, where thin film growth of the dielectric material takes place to a specific pre-determined thickness, as in the case of rugate coatings, in order to produce a specific wavelength transmittance profile for that particular comb filter. Because of the requirement to match all of the band wavelength positions on both filters, the fabrication of complementary comb filter pairs is extremely difficult. For example, the deposition thickness and pattern of the thin films of the respective comb filters may have been improperly done, and therefore, the overall combined transmittance profile of the complementary pair is thus unacceptable. This problem leads to increased material costs due to rejected comb filters, delays in manufacturing, and increased labor costs.
Therefore there exists a need for new and improved complementary comb filter pairs, and simple, inexpensive, and accurate methods for making same.